This invention is directed to the diagnosis and detection of gross or macroinstabilities in a magnetically-confined fusion plasma device. Detection is performed in real time, and is prompt such that correction of the instability can be initiated in a timely fashion.
A plasma in a magnetic field has a tendency to be unstable; as a result, it can break up and escape from confinement by the field. Plasma instabilities are due basically to the presence of electrically charged particles; the electric and magnetic fields produced by their motions cause the particles to act in a collective, or cooperative manner. An example of such collective action is the drift of a plasma in a non-uniform magnetic field. Similar collective effects give rise to plasma instabilities. These instabilities fall into two broad categories, called "gross hydromagnetic instabilities" and "more localized microinstabilities". Suppose, for example, a small displacement of a plasma occurs in a magnetic field; if the system reacts in such a way as to restore the original condition, then it is stable. In the case of a hydromagnetic (or gross) instability, however, the plasma does not recover, but the displacement increases rapidly in magnitude. The whole plasma may then break up and escape even from a strong magnetic field.
Microinstabilities, as the name implies, are on a small scale compared with the dimensions of the plasma. As a rule, these instabilities do not lead to complete loss of confinement, but rather to an increase in the rate at which the plasma diffuses out of the magnetic field. This invention is directed to the former type of instabilities, the gross or macroinstabilities.
The term "plasma instability" refers to any cooperative plasma motion that can regenerate itself, starting from normal levels of random flunctuations or of plasma irregularities, in a time short compared to collision processes in the plasma. Thus, plasma instability can connote motions that range all the way from a gross motion of the plasma as a whole across a confining field, to high frequency, short-wavelength isolations of the plasma accompanied by intense flunctuating electric fields, but perhaps by little transport.
By their nature, gross instabilities are slow growing, that is their growth rate is much less than the ion cyclotron frequency, and involve wave lengths that are generally large compared to particle orbit diameters. They owe their origin to a simple circumstance: if a magnetically confined plasma can convert some of its internal kinetic energy to a directed motion by distorting or moving in some direction across the confining field, this process will occur. Though they are slow growing, compared to fine-scale instabilities, the effects of gross instabilities on confinement are the most catastrophic of all. Their growth time scale is of the order of the transit time of an ion through the confinement chamber--i.e., of the order of microseconds in fusion plasmas situations.
This invention is directed to disruptive instabilities which are characterized by a sudden, large disturbance that develops very rapidly (typically tens to hundreds of microseconds in ohmically heated plasmas), and is accompanied by hard x-ray bursts, flattening of a current profile with expulsion of poloidal flux and prominent negative voltage spikes, loss of energy, and shift of the plasma column to smaller major radius. Small disruptions may repeat several times; large ones may terminate the discharge.
Major plasma disruptions present a formidable design problem because of the rapid release of both thermal and magnetic energy. In general, disruptions are expected to occur at the limits of operation in current and/or density, but the mapping of an essentially disruption-free operating machine has been an empirical exercise for each device.
The design impact of a sudden loss of confinement is severe. Indeed, for all large tokamaks, major disruptions play an important role in first wall design. References which describe the engineering overdesign required by the occurrence of plasma current disruptions in Tokamaks are: "Mechanical Engineering Aspects of TFTR, J. C. Citrolo, Princeton Plasma Physics Laboratory Report No. PPPL-1988 [983], and Engineering Asppects of Disruption Current Decay", J. G. Murray, ORNL/FEDC-83/S(1983). Major disruptions also play a significant role in determining the requirements for vessel clean-out, plasma control, vertical field coil placement, and they even impact toroidal field coil design (when superconducting coils are employed). For example, a major disruption on a typical long pulse, d-t burning tokamak will cause the deposition of 100 to 200 MJ of plasma energy onto the surface of the first wall on a time scale short compared to the thermal diffusion time, so as to cause rapid heating and subsequent vaporization of substantial quantities (of the order of kilograms) of wall material.
It is obviously beneficial to provide a mechanism for control of major disruptions. In addition to the engineering advantages associated with the reduced thermal loading to the first wall and to the limiter, a reduction of severe JxB forces on coils, the prevention of the major disruption also allows lower q operation, which, if .beta..sub.p is limited by balooning, implies a higher beta operation. While some success has been achieved in disruption control, the techniques employed also present major design problems when examined in a reactor context.
It is therefore an object of the present invention to predict the occurrence of a major plasma disruption in real time, with enough advance lead time to allow corrective plasma control actions to be taken.
It is another object of the present invention to provide a plasma diagnostic technique which reveals plasma instability precursors, is easy to operate, and which can be implemented with a minimum-size system comprised of standard laboratory devices.